Modern integrated circuits contain thousands of semiconducting devices on a single chip, and as the device density of a given chip increases more levels of metallization are required to interconnect the devices. Moreover, within a given metal level the horizontal distance separating metallization lines must be reduced in order to minimize the chip's size, as the device density increases. At the same time, metallization resistance must be minimized in order to meet the chip's speed and performance requirements. Traditionally, aluminum and aluminum alloys have been used throughout the semiconductor industry for integrated circuit metallization. The semiconductor industry's continuing demand, however, for integrated circuits with ever increasing device densities requires new metallization materials having higher electrical conductivity and higher electromigration resistance than can be obtained with traditional aluminum based metallization. Therefore, several alternatives to aluminum based metallization have been proposed for use in advanced integrated circuits.
Copper, silver, and gold are some of the alternative metallization materials that have been proposed. Their electrical conductivities are higher than aluminum, or its alloys, and at the same time their resistance to electromigration is higher than traditional aluminum based metallization. Moreover, copper can be selectively deposited and therefore it does not have to be patterned with a separate processing step. These well-known advantages make these metallization materials attractive for advanced integrated circuits.
Many of these alternative metallization materials, however, rapidly diffuse through dielectric materials, and thus they may adversely effect the integrated circuit's reliability and functionality. Therefore, barrier layers between the metallization and the dielectric layers are often required to prevent diffusion. In addition, metallization formed by selective deposition requires a nucleation layer or seed layer to promote preferential deposition. Unfortunately, barrier layers and seed layers often limit reductions in the horizontal distance separating adjacent metal lines. Therefore, the ability to achieve integrated circuits with high device packing densities is limited with these alternative metallization materials. Accordingly, a need exists for a metallization process that allows the fabrication of integrated circuits with high device packing densities.